1. Technical Field
The present invention relates to a safety stock amount calculation method and a safety stock amount calculation device, and more particularly to, a technique effectively applied to a safety stock amount calculation processing in a system in which inventory management is performed based on a projected inventory.
2. Background Art
A method of calculating a safety stock level based on a standard deviation of past shipments, a lead time L, a safety coefficient k or the like has been known. In this method, a safety stock amount SS is calculated by the following
                                              ⁢                  SS          =                      k            ⁢                                                  ⁢                                          σ                ′                            ⁢                                                          (                                                σ                  ′                                =                                                                            stock                      ⁢                                                                                          ⁢                      adjustment                      ⁢                                                                                          ⁢                      period                      ×                      shipment                      ⁢                                                                                          ⁢                      frequency                                                        ×                  standard                  ⁢                                                                          ⁢                  deviation                  ⁢                                                                          ⁢                  of                  ⁢                                                                          ⁢                  demand                  ⁢                                                                          ⁢                  σ                                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
Where, the demand standard deviation σ is calculated from daily demand amount and shows variation of the daily demand with respect to an average demand. The fact that collective stochastic phenomena generally approximates to a normal distribution (central limit theorem) is known, and a demand amount is considered to also conform to a normal distribution in most cases. Here, assuming that a daily standard deviation is σ, a standard deviation per N days is represented as follows due to additivity of variance:√{square root over (N)}×σ
Therefore, in the [Equation 1], the square root of (inventory adjustment period×shipment frequency) is used.
The safety coefficient k represents the level of a permissible stockout rate and is determined depending on how much uncertainty such as demand fluctuation or prediction error is taken into consideration. The safety coefficient k is set based on a target ratio of service S. For example, a safety coefficient of 1.65 is obtained from a normal distribution table of quantity demanded when a ratio of service is 95% (95% of demand is met; permissible stockout rate 5%).
The inventory adjustment period is the length of a period during which a response to the customer's order must be made with a quantity corresponding to a single order. The inventory adjustment period is the lead time L itself in the case of an inventory management according to an order-point system, whereas in the case of an inventory management according to a periodic ordering system, the inventory adjustment period becomes a period obtained by adding an ordering cycle M to the lead time L. The shipment frequency denotes the number of orders made in the inventory adjustment period. In the case where orders are made three times per ten days, for example, a value of 0.3 or the like is set as the shipment frequency. When the demand standard deviation σ is calculated in units of a week or month, the time unit of the lead time L or ordering cycle M is correspondingly changed.
Recently, in companies' core information processing systems, an ERP (Enterprise Resource Planning) package that processes information related to various management tasks including inventory management, such as, accounting management, product management, sales management or personnel management distributed within the company in an integrated manner is gaining acceptance. In the ERP package, task modules such as inventory management are designed with an integrated database having daybook-type features as a central core, wherein each of the task modules independently functions in general. Of the task modules, the inventory management module adopts a so-called “MRP” (Material Requirements Planning). In the MRP, a production plan is drawn up with the attention paid to the required amount and period of materials based on a relationship between parts of the product and lead time, and required items or the like are ordered based on a projected inventory which is a predicted value of projected inventory amount.
In the ERP package described above, inventory management is practiced as follows. That is, calculation methods of the reorder point and constant order quantity are previously defined in the case of an order-point system, and a calculation method of the order quantity is previously defined in the case of a periodic ordering system, and a user side manages a reorder task by appropriately inputting numeric values. The reorder point or order quantity is determined using the aforementioned safety stock amount. In the order-point system, “reorder point=minimum stock=average demand during inventory adjustment period (during lead time L)+safety stock amount” is satisfied. On the other hand, in the case of the periodic ordering system, “order quantity=commodity amount to be used during inventory adjustment period+safety stock amount−current stock amount−current order remaining amount” is satisfied.
However, the safety stock amount calculation method as described above is a method that calculates a safety stock amount obtained in the case where orders are made with reference to the stock amount and reorder point in the time when the orders are being made. Therefore, the above safety stock amount calculation method does not always adapt to a projected inventory-based system. For the above reason, when the calculation values obtained according to the conventional method are used in the system such as the ERP package in which inventory management is carried out based on a projected inventory, the safety stock amount may become too small or too large, with the result that it is difficult to set a proper reorder point or order quantity. On the EPR software, therefore, calculated values obtained according to the conventional method are input as the safety stock amount with the knowledge that accuracy may be decreased, or the values obtained by appropriately correcting calculated values based on the seat-of-the-pants estimate of a person in charge are input as the safety stock amount, failing to bring out system capability. Naturally, it is of high importance to overcome these problems.
An object of the present invention is to provide a safety stock amount calculation method and a safety stock amount calculation device that conform to the system that carries out inventory management based on a projected inventory.